We have demonstrated that ethyl methane sulfonate (EMS) exposure of single Chinese hamster ovary (CHO) cells will produce colonies mosaic for glucose-6 phosphate dehydrogenase (G6PD) activity and also colonies uniformly lacking in activity (pure mutant). The unstained cells in these mosaic colonies appear in various sectored patterns which are remarkably similar to patterns observed in yeast and bacteria after similar treatments. This is a new phenomenon for a somatic mammalian cell culture system and we propose to investigate the basis of this phenomenon and use it as a tool to study mutagenesis in mammalian cells. Pure mutant clones may be caused by death of the wild-type cell after the first cell division (lethal sectoring) leaving the mutant cell to grow into a colony. This possibility will be investigated by measuring the amount of lethal sectoring occurring at various mutagen doses, and correlating this with the ratio of pure:mosaic colonies occurring at these doses. Pure mutant and certain mosaic colony type may be caused by multiple-mutational events at the G6PD loci. This possibility will be investigated by examining subclones derived from the same mutant colony for differences in G6PD electrophoretic mobility or alterations in enzyme kinetic parameters. The relationship of DNA repair to mosaic phenomenon will be investigated by examining the following sectoring parameters using mutagenic agents which produce different types of DNA damage: 1) change in mosaic:pure mutant ratio with dose, 2) distribution of sector sizes, 3) timing of the mutational fixation event, 4) production of replicating instabilities and 5) lethal sectoring. These parameters will also be examined using various mutagen sensitive and repair defective CHO mutants and also drugs such as caffeine which inhibit DNA repair. The process of mutagenesis has important health implications since current evidence strongly suggests that carcinogenic agents exert their effect through mutagenic reactions with DNA.